BY THE OPTIMIST DAILY EDITORIAL TEAM

When the sun sets, solar panels sadly become unproductive. For decades, that daily pause has underscored one of renewable energy’s biggest challenges: how to store sunlight efficiently for use after dark.

Researchers at UC Santa Barbara believe they have found an elegant answer. No, not in bulky battery packs or vast grid systems, but inside a small, carefully engineered molecule.

In a study published in Science, Associate Professor Grace Han and her team describe a new material that captures solar energy, locks it into chemical bonds, and releases it later as heat on demand. The innovation belongs to a growing field known as molecular solar thermal (MOST) energy storage, and it may offer a lightweight, recyclable alternative to conventional batteries.

“The concept is reusable and recyclable,” said Han Nguyen, a doctoral student in the Han Group and lead author of the paper.

To explain it, Nguyen reaches for an everyday analogy. “Think of photochromic sunglasses. When you’re inside, they’re just clear lenses. You walk out into the sun, and they darken on their own. Come back inside, and the lenses become clear again,” Nguyen said. “That kind of reversible change is what we’re interested in. Only instead of changing color, we want to use the same idea to store energy, release it when we need it, and then reuse the material over and over.”

Inspired by DNA

The breakthrough began with a surprising source of inspiration: DNA. The team focused on a molecule known as pyrimidone, structurally similar to components found in DNA that undergo reversible changes when exposed to ultraviolet light. By engineering a synthetic version of this structure, the researchers created a compact molecule capable of storing and releasing energy repeatedly without degrading.

To better understand how the molecule could remain stable while holding onto stored energy for years, the group collaborated with Ken Houk, a distinguished research professor at UCLA. Using computational modeling, they analyzed the molecule’s structure and energetic behavior.

“We prioritized a lightweight, compact molecule design,” Nguyen said. “For this project, we cut everything we didn’t need. Anything that was unnecessary, we removed to make the molecule as compact as possible.”

That minimalist approach paid off. The final molecule is small, efficient, and durable, which are all key traits for scalable clean energy technologies.

A rechargeable battery for heat

Unlike solar panels, which convert light into electricity, this system converts sunlight into chemical energy. The molecule behaves like a tiny mechanical spring. When sunlight strikes it, the structure twists into a high-energy configuration and remains locked in that strained state.

It stays that way until triggered by a small amount of heat or a catalyst, and then it snaps back into its relaxed form, releasing the stored energy as heat.

“We typically describe it as a rechargeable solar battery,” Nguyen said. “It stores sunlight, and it can be recharged.”

The numbers are striking. The molecule achieves an energy density of more than 1.6 megajoules per kilogram. By comparison, a standard lithium-ion battery delivers roughly 0.9 megajoules per kilogram. That means this liquid system stores nearly double the energy per unit weight and does so in a recyclable, reusable format.

From lab theory to boiling water

High energy density is impressive on paper. But the UCSB team wanted proof that the stored energy could deliver meaningful, real-world heat.

In laboratory demonstrations, the researchers showed that the material could release enough energy to boil water under ambient conditions, reaching an undeniable milestone in the MOST field.

“Boiling water is an energy-intensive process,” Nguyen said. “The fact that we can boil water under ambient conditions is a big achievement.”

That proof of concept opens the door to practical uses. Off-grid campers could one day carry compact systems that store solar heat during the day and release it at night. Homes might integrate roof-mounted solar collectors that charge the liquid during daylight hours, then circulate it through water heaters after sunset.

Because the molecule is soluble in water, it could potentially be pumped through closed systems, making storage and distribution relatively straightforward.

“With solar panels, you need an additional battery system to store the energy,” said co-author Benjamin Baker, a doctoral student in the Han Lab. “With molecular solar thermal energy storage, the material itself is able to store that energy from sunlight.”

A promising step for renewable energy

Renewable energy breakthroughs often focus on electricity storage. But heat accounts for a large share of global energy demand, from residential water heating to industrial processes.

By creating a compact, high-density molecular system that can capture, store, and release heat on demand, the UCSB team offers a new pathway for solar energy that may reduce reliance on traditional batteries and heavy infrastructure.

The work was supported by the Moore Inventor Fellowship, which Han received in 2025 to advance the development of what the team calls “rechargeable sun batteries.”

The sun may still set each evening. But with innovations like this, its energy could linger long after the daylight fades.

Source study: Science—Molecular solar thermal energy storage in Dewar pyrimidone beyond 1.6 MJ/kg

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